Naked-eye detection of Pd2+ ion using a highly selective fluorescent heterocyclic probe by “turn-off” response and in-vitro live cell imaging

•The new diazaheterocycle, 3-(1-isoquinolinyl)imidazo[5, 1-a]isoquinoline (IQ) is selective for sensing palladium(II) ion.•Blue fluorescence exhibited by IQ upon exciting with 359 nm light gets “turn-off” upon adding Pd(II) ion.•The probe IQ shows a remarkably low limit of detection of 0.210 μM.•A 5 μM solution of IQ showed low cytotoxicity towards L929 cells thus Pd2+ ion can be detected in living cells.